The basic digital-to-analog converter (DAC) is disclosed in copending patent application Ser. No. 777,235 which was filed Mar. 11, 1977, by Adib R. Hamade and Sam S. Ochi, is now abandoned, and is a continuation of application Ser. No. 608,873 filed Aug. 29, 1975, which is now abandoned. A resistor ladder and associated switching tree are shown for digital-to-analog (D/A) and analog-to-digital (A/D) conversion applications. An MOS implementation is also disclosed.
The resistor ladder and switching tree form of DAC has proven to be very useful and amenable to integrated circuit (IC) fabrication using modern large scale integration (LSI) techniques. The resistor ladders have been fabricated in the form of diffused resistors, ion implanted resistors and deposited metal film resistors. The switches in the switching tree can be PMOS or NMOS switches or in the form of CMOS transmission gates. Single-chip, 8-bit, A/D converters using the successive approximation register (SAR) approach have become commercially available at low cost.
The main problem in 8-bit and higher devices is in the large number of resistors and switches that must be used and consequently, the large area required in LSI chips. Not only does the large parts count increase chip size it also reduces fabrication yield and reliability.